"I'm on a horse." Arguably, it's the most recognized and repeated catchphrase since Beavis (of "Beavis and Butthead" fame) pulled his shirt on his head and demanded "TP" for his … well, you know.

You know that Beavis line just like you know, "I'm on a horse" is from the insanely successful "The Man Your Man Could Smell Like" Old Spice ad campaign. Thanks to seamless scene changes, Harlequin romance scripting and Isaiah Mustafa's over-the-top bravado, "I'm on a horse" is ingrained in our pop culture consciousness. Unfortunately for Old Spice, that campy cache hasn't translated to increased sales.

The official "I'm on a horse" video is at 12.2 million views and growing. Last week, new Old Spice spots occupied a record-setting four positions on the "Top 10 Viral Video Ad Campaigns Chart," compiled by Visible Measures, an industry-tracking agency.

"The Return of the Man Your Man Could Smell Like" — in which dude's on a motorcycle instead of a horse — hit No. 1 last week with 2.9 million views on YouTube. Old Spice "Odor Blocker" —in which other dude (Terry Crews aka Chris Rock's TV dad) wears hot pants and yells a lot — hit No. 2 with 2.4 million views. The original "I'm on horse" ad held tight at No. 3, with 1.3 million views. And at No. 10, Old Spice's "Where Freshness Smells From" — in which yet a third dude is way too reminiscent of "American Psycho" — came in at No. 10. (You can see all the videos here.)

But sales of "Red Zone After Hours Body Wash" (the specific Old Spice product Mustafa is on a horse shilling — yeah, I didn't know that, either) have fallen by 7 percent in 2010, says WARC, another marketing tracking agency.

Awesome overload distracting from the ad's core message? Maybe so, Don Draper. "The Man Your Man Could Smell Like" won the Film Grand Prix at the 2010 Cannes Lions International Advertising Festival. And as WARC notes, 86 percent of the brands that took prizes at events such as Cannes saw increased sales.

Feminist blog Jezebel has a few theories, all of which add up to a plausible point about how even the most successful viral video can fail in its ultimate mission: To sell the product. Here's the truncated summary:

"The funny ads actually gave the product embarrassing or negative connotations.

The women who were supposed to buy the stuff for their supposedly smelly male significant others were either purchasing other soap products or, more likely, not buying body wash for their dudes at all.

A lot of us are still going to forever associate it with our fathers (and grandfathers) and stay as far away as possible."